How can I add Lens Flares/Halos?
lens-flare shader

There is an OSL lens-flare shader by iReboot (aka Dylan Whiteman) which may interest you. 

From his blog post referring to a lens-flare shader originally written for the Renderman language by Larry Gritz &amp; Tony Apodaca of pixar, iReboot writes:


  I decided to try and port this to OSL - (...) I added a few features, such as
  user definable images for lens-flare elements and have got to a point
  where the shader can now be released.(...) - along with video tutorial links